This invention relates to a secondary air supply system which is provided in an exhaust system of an internal combustion engine at a location upstream of a three-way catalyst for supplying secondary air into the exhaust system to thereby enhance the purifying efficiency of the three-way catalyst.
Conventionally, a failure detection system for a secondary an supply system of this kind has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-111256, which is provided for an internal combustion engine having an exhaust passage, and the secondary air supply system, an air-fuel ratio sensor, and a catalyst, which are arranged in the exhaust passage in the order mentioned as taken from the upstream side toward the downstream side. According to the failure detection system, when the engine is in a particular operating condition in which the secondary air supply system should be operated to supply secondary air into the exhaust passage, i.e. when the engine is idling or decelerating, if the concentration of oxygen remaining in exhaust gases in the vicinity of the air-fuel ratio sensor, which is detected by the sensor, is low (i.e. the air-fuel ratio is rich), it is determined that there is abnormality in the secondary air supply system, such as clogging thereof, which causes shortage in the supply of oxygen into the exhaust passage.
In order to positively determine abnormality of the secondary air supply system by the above proposed failure detection system, it is prerequisite that a rich air-fuel mixture should be supplied to the engine. However, in actuality, when the engine is in such a particular operating condition, the air-fuel mixture is not necessarily rich. More specifically, particularly when the engine is decelerating, the supply of fuel is normally interrupted (fuel cut), so that only fuel adhering to the inner wall surfaces of the intake pipe etc. is supplied to the cylinders, and hence combustion of fuel (explosion) in the cylinders does not necessarily take place. If the combustion does not occur, the concentration of oxygen remaining in the resulting exhaust gases (which correspond to exhaust gases produced by combustion of a lean air-fuel mixture) becomes high. In such a case, even if the secondary air supply system suffers from abnormality which prevents secondary air from being supplied into the exhaust passage, it can be determined that the secondary air supply system is normally functioning.
Another system for detecting failure in a secondary air supply system has been proposed by Japanese Provisional Patent Publication (Kokai) No. 63-212750, which determines that there occurs abnormality in the secondary air supply system (e.g. the valve of the system is held open), if an O.sub.2 sensor disposed in the exhaust system at a location downstream of the secondary air supply system continually detects a lean mixture over a predetermined time period when the engine is in a high-load increased-fuel-supply operating condition in which secondary air is not supplied to the exhaust passage.
According to this conventional system, insufficient oxygen supply is not detected but excessive oxygen supply is detected to thereby determine abnormality in the secondary air supply system, which may be caused by the faulty valve which is held open. Therefore, even if the secondary air supply system is faulty due to failure of the valve to open, the failure detection system cannot determine that the secondary air supply system is faulty, which can result in an abnormal rise in the catalyst temperature and hence degradation of the catalyst operation.